1. Field
The presently disclosed subject matter relates to an optical deflector used as a scanner in a projector, a bar code reader, a laser printer, a laser read amplifier, ahead-up display apparatus or the like.
2. Description of the Related Art
Recently, in a projector or the like, a spotlight is deflected by an optical deflector and then, is projected on a screen. Such an optical deflector is a micro electro mechanical system (MEMS) device manufactured by semiconductor manufacturing processes and micro machine technology.
A first prior art one-dimensional optical deflector is constructed by a circular mirror, a support frame surrounding the circular mirror, a pair of torsion bars connected between the support frame and the circular mirror, piezoelectric actuators provided between the support frame and the torsion bars for vibrating (rocking) the circular mirror through the torsion bars with respect to an axis of the circular mirror, and two semi-ring-shaped piezoelectric sensors provided at a circumference of the circular mirror for sensing rocking vibrations caused by the piezoelectric actuators (see: FIG. 1 of JP2011-150055).
The above-described first prior art one-dimensional optical deflector has, however, the following problems:
a) Since the semi-ring-shaped piezoelectric actuators are also rocked in association with rocking of the mirror, the resonant frequency thereof would be decreased.
b) Since conductive layers located from the semi-ring-shaped piezoelectric sensors on the circular mirror through the torsion bars to pads on the support frame are very long, the resonant frequency of the circular mirror is also affected by the weight of the conductive layers, so that the resonant frequency of the circular mirror would fluctuate from an optimum design value.
c) When one of the semi-ring-shaped piezoelectric sensors is expanded, and the other semi-ring-shaped piezoelectric sensor is retracted, the sense voltages of the two semi-ring-shaped piezoelectric sensors opposite in phase to each other would be compensated for by each other, thus reducing the entire detected sense voltages of the semi-ring-shaped piezoelectric sensors.
In a second prior art one-dimensional optical deflector, one piezoelectric sensor is provided between one of the torsion bars and one of the piezoelectric actuators on the same beam as that of the one of the piezoelectric actuators, thus solving all the problems of the above-described first prior art one-dimensional-optical deflector (see: FIGS. 16, 17 and 18 of JP2010-197994A & US2010/0195180A1).
In the above-described second prior art one-dimensional optical deflector, however, since the width of the piezoelectric sensor is the same as that of the piezoelectric actuators, the accuracy of the sense voltage of the piezoelectric sensor with respect to a resonant frequency is insufficient. As a result, even when the frequency of sinusoidal-wave drive voltages for the piezoelectric actuators is controlled by the sense voltage of the piezoelectric sensor, a resonant state cannot be realized. This will be discussed in detail later.
Note that, if a resonant state cannot be realized, it would be required to increase the drive voltages for the piezoelectric actuators in order to obtain a desired deflection angle of the circular mirror.